1. Field of the Invention
The invention relates to the technology of making optical WDM (Wavelength-Division-Multiplexing) multiplexer or demultiplexer using triple-fiber ferrule.
2. The Related Art
The future communication networks demand ever increasing bandwidth. By transmitting several channels in a single optical fiber at different wavelengths, WDM can greatly enhance the transmission capacity of the optical fiber communication networks. A device that combines different wavelength channels into one fiber is a so-called multiplexer, and a device that divides the multiplexed channels into individual ones is a so-called demultiplexer. A variety of technologies have been exploited to develop high performance WDM multiplexer/demultiplexer, including fiber Bragg granting, optical integrated circuit, fused fiber Mach-Zander interferometer, interference thin film coating technology etc. An international standard wavelength grid has been suggested by the ITU (International Telecommunication Union) for the center wavelengths of the WDM channels. The prior arts use a Y-branch structure of three conventional fiber optic collimators to tune the filter center wavelength to the ITU grid. The incident wavelength of such a multiplexer/demultiplexer is made to coincide with a desired ITU wavelength. The disadvantage in the prior arts is mainly in the difficulty of reducing its size to achieve a robust and compact structure thereof. An approach has been disclosed in the copending application Ser. No. 09/255,047 filed on Feb. 22, 1999, now U.S. Pat. No. 6,246,812, in which a robust and compact structural multiplexer/demultiplexer relative to the prior arts is presented by means of V-groove ferrules. Anyhow, it is still desired to have a smaller, less-components structure for the multiplexer/demultiplexer.
Therefore, an object of the invention is to provide a multiplexer/demultiplexer with a half-sized structure thereof relative to that disclosed in the aforementioned copending application so as to result in a compact, robust structure thereof, low cost and flexibility in manufacturing, and high stability in hazardous environment. A copending application Ser. No. 09/576,756 filed May 23, 2000, now U.S. Pat. No. 6,400,862, discloses an approach to achieve this goal. Anyhow, the invention is to provide an improvement to such approach""s application.
The retroreflective multi-port fiber optic device with triple-fiber ferrule in the present invention is designed for use as WDM multiplexer/demultiplexer. One embodiment of the invention using a GRIN rod lens to couple the light. The triple-fiber ferrule is attached to the angled facet of the GRIN rod lens and the WDM filter is attached to the opposite side of the lens. A reflective mirror is aligned and attached next to the other side of the WDM filter.The selected one from a series of triple-fiber ferrules isused to couple light in and out of the device and tune the thin film filter center wavelength to the ITU grid. The light coupled in from the common input pigtail fiber of the triple-fiber ferrule will be collimated and transmitted onto the WDM filter. The part of the in-pass-band light will transmit through the WDM filter, then retro-reflected back by the mirror and transmit through the WDM band-pass filter again. The retro-reflected in-pass-band signal is then coupled into the transmission output port of the triple-fiber ferrule as a demultiplexed channel.
Same incidence angles to the WDM filter are required for the transmitted and retroreflected in-pass-band light beam to achieve the best performance from the WDM filter. In order to maintain the same incidence angle onto the WDM filter. The three pigtail fiber centers in the triple-fiber ferrule are located on a round virtual circle with the connecting segment of the common input pigtail fiber and the reflection output pigtail fiber as a diameter. The center of this virtual round circle is also substantially coincident with the lens optical axis. Typically the three fibers are located at the three tips of a T-shape. Particularly selected fiber spacing is used to match the specific WDM filter. The invention has especially low cost and high flexibility in achieving a series of different fiber spacing for tuning the center wavelength of a band-pass WDM filter to coincide with that of the ITU grid. The part of the out-pass-band light is directly reflected back by the WDM filter and coupled into the reflection output pigtail fiber of the triple-fiber ferrule to be transmitted to the next stage for demultiplexing other channels. All parts of the device are bounded together by adhesives such as epoxy after alignment.
The features of the invention further includes provision of a series of three-fiber ferrules which respectively define three-fiber assemblies with different virtual circle diameters. Under this condition, the technician can select the specific one from this series of three-fiber ferrules to precisely meet/coincide with the filter to reach the desired center wavelength required by ITU.
The invention results in especially low cost and high flexibility in achieving a dense series of different fiber spacings for tuning the center wavelength of a given band-pass WDM filter to coincide with that of the ITU grid.